The reduction of nitric oxide emissions of a combustion engine that works with oxygen surplus, in particular a diesel combustion engine can take place with the aid of the so-called selective catalytic reduction technology (SCR). A reduction of nitric oxides into nitrogen and hydrogen is thereby carried out, whereby gaseous ammoniac or ammoniac in aqueous solution or urea in aqueous solution are used as reducing agent. The urea serves thereby as ammoniac carrier. With the aid of the metering system in front of a hydrolysis catalytic converter the reducing agent is injected into the exhaust gas pipe of the combustion engine. It is converted in the hydrolysis catalytic converter with the aid of the hydrolysis into ammoniac, which then further reduces the nitric oxides in the exhaust gas in the main SCR-catalytic converter, which is also called DENOX-catalytic converter. The main components of such a NOx-reduction system are a reducing agent tank, a pump, a pressure regulator, a pressure sensor and a metering valve. The pump advances the reducing agent that is stored in the reducing agent tank to the metering valve, with which the reducing agent is injected into the exhaust gas current upstream of the hydrolysis catalytic converter. The metering valve is thereby controlled by signals of a control unit, for example the control unit of the combustion engine, in order to supply a specific actually required amount of reducing agent. Substances that are present in aqueous solution and releasing ammoniac, such as urea, are preferably used, because they store reducing agent significantly easier and are easier to handle than for example gaseous ammoniac. Furthermore the ability to advance and meter this solution is technically significantly easier than for example the advancing and metering of gaseous substances. A disadvantage of the aqueous solution of for example urea is that the reducing agent solution has a freezing point of −11° C. Therefore devices have to be provided for heating the reducing agent in the reducing agent tank.
The reducing agent amount is advanced at such reduction systems by a pipe from the reducing agent tank to the metering valve. The pipe pressure is regulated to a constant value with the aid of a controlling of the pump. A significant requirement therefore is the proper functioning of the pump, which is necessary to be able to keep the default nominal pressure in the system stabile.
The invention is therefore based on the task to provide a procedure for checking the functionality of a pump engine of a reducing agent pump, which can be realized by simple means.